1. Field of the Invention
The present invention relates generally to intraoperative neural monitoring and, more particularly, to apparatus and methods for intraoperative neural monitoring involving monitoring of the spinal cord using motor evoked potentials elicited by electrical stimulation.
2. Brief Discussion of the Related Art
Intraoperative neural monitoring involving intraoperative monitoring of the spinal cord has become accepted as an effective means to avoid neural deficits in patients undergoing various types of surgical procedures in which the spinal cord is at risk of injury. By monitoring the integrity of the spinal cord motor tracts during surgery, impairments in motor function may be detected before they become irreversible and while there is sufficient time to institute corrective measures.
Spinal cord monitoring has traditionally relied on the Stagnara wake-up test, which is ordinarily performed at the conclusion of a surgical procedure and thusly does not provide an early indication of spinal cord dysfunction. Wake-up testing is limited to evaluating gross motor function and fails to identify more subtle spinal cord impairments. Oftentimes administration of the wake-up test is compromised by anesthetic influences. The wake-up test depends on a subjective assessment of a patient's motor responses, and is usually of little value in patients whose motor responses are already impaired by preexisting neural deficits. Additional disadvantages of the wake-up test include the risks of air embolism and self-extubation.
Sensory evoked potentials (SEPs) have also been used for intraoperative spinal cord monitoring, primarily to monitor the dorsal medial tracts within the spinal cord. SEPs are ascending motor volleys elicited by stimulating a peripheral nerve, commonly the posterior tibial nerve at the ankle (medial malleolus), and conducted primarily through the dorsal columns of the spinal cord. SEPs may be detected and recorded as waveforms at various anatomical locations along the nerve tract including peripherally (e.g. popliteal fossa), cervically and cortically. Medically significant changes in amplitude and latency of SEP waveforms during surgery may be indicative of surgically-induced sensory deficits (parathesia). However, it is possible for motor deficits to develop intraoperatively despite the lack of medically significant changes in recorded SEPs, i.e. false negatives. In addition, in some patients it may be difficult or not possible to obtain SEP readings intraoperatively. Being low amplitude, SEP responses require averaging over time such that the readings obtained from SEPs are not as close to real-time as would be desirable. Routine intraoperative spinal cord monitoring using SEPs cannot effectively spatially resolve the loss of certain nerve roots, such as the lumbosacral root, which optimally requires electromyographic (EMG) responses from muscles enervated by the nerve roots.
A more recent form of spinal cord monitoring that addresses many of the disadvantages of the wake-up test and SEPs involves monitoring the spinal cord motor tracts using motor evoked potentials (MEPs). Transcranial electrical stimulation to stimulate the motor cortex has been proposed for eliciting MEPs, which are descending motor volleys conducted along the motor pathways of the spinal cord. The motor cortex can be stimulated non-invasively through an intact skull with electrical current of sufficient magnitude applied via appropriately placed stimulating electrodes. MEPs can be recorded at various anatomical locations including the spine, innervated muscles of the upper and lower extremities (myogenic), and peripheral nerves (neurogenic). Medically significant changes in recorded MEPs during surgery may be indicative of surgically-induced motor deficits (paraplegia), and MEPs are believed to be more sensitive to certain types of spinal cord trauma than SEPs. MEPs recorded from the spinal cord reflect the functional integrity of the corticospinal tract, and MEPs recorded from limb muscles reflect the functional integrity of the motor system from the cerebral cortex to beyond the neuromuscular junction. By stimulating the motor cortex on both sides of the patient's body and recording myogenic MEPs (compound muscle action potential) in muscles on both sides of the patient's body, unilateral neural deficits can be differentiated.
Although magnetic stimulation of the motor cortex can be used to elicit MEPs, transcranial electrical stimulation is generally preferred because magnetic motor evoked potentials are more sensitive to anesthetic-induced depression than electrical motor evoked potentials. Although anesthetics reduce synoptic efficacy and decrease cortical excitability as well as the excitability of spinal motoneurons and interneurons, repetitive or multipulse transcranial stimulation with electrical pulses of sufficiently high current can still elicit MEPs by enhancing temporal summation of the descending input on spinal motoneurons. It is also possible to elicit MEPs by direct electrical stimulation of the spinal cord using epidural electrodes or needle electrodes placed near or in the vertebral bodies with recording accomplished in muscles, nerves and/or the epidural space.
MEPs are large amplitude responses that do not require signal averaging, such that reporting may be accomplished essentially real-time. MEPs provide fast, practical and reliable qualitative information on the functional integrity of the motor tracts of the spinal cord. Because MEPs and SEPs are conducted in different spinal cord pathways having different blood supplies, MEPs may be present in patients when SEPs are absent or ill-defined. MEP monitoring thusly makes it possible to monitor the spinal cord in patients for whom SEP signals are unobtainable. Furthermore, MEPs may better reflect the integrity of the anterior spinal cord than SEPs.
Representative discussions of transcranial electrical stimulation to elicit MEP responses for monitoring the spinal cord during spinal surgery are set forth in “Cotrel-Dubousset Instrumentation in Children Using Simultaneous Motor And Somatosensory Evoked Potential Monitoring” by Stephen, Sullivan, Hicks, Burke, Woodforth and Crawford, “Threshold-level repetitive transcranial electrical stimulation for intraoperative monitoring of central motor conduction” by Calancie, Harris, Brindle, Green and Landy, “A comparison of myogenic motor evoked responses to electrical and magnetic transcranial stimulation during nitrous oxide/opioid anesthesia” by Ubags, Kalkman, Been, Koelman, and de Visser, “lntraoperative monitoring of spinal cord function using motor evoked potentials via transcutaneous epidural electrode during anterior cervical spinal surgery” by Gokaslan, Samudrala, Deletis, Wildrick and Cooper, “lntraoperative spinal cord monitoring for intramedullary surgery: an essential adjunct” by Kothbauer, Deletis and Epstein, “Improved amplitude of myogenic motor evoked responses after paired transcranial electrical stimulation during sufentanil/nitrous oxide anesthesia” by Kalkman, Ubags, Been, Swaan and Drummond, “Repetitive vs. single transcranial electrical stimulation for intraoperative monitoring of motor conduction in spine surgery” by Haghighi and Gaines, “Monitoring of motor evoked potentials with high intensity repetitive transcranial electrical stimulation during spinal surgery” by Haghighi, “Monitoring scoliosis surgery with combined multiple pulse transcranial electric motor and cortical somatosensory-evoked potentials from the lower and upper extremities” by MacDonald, Zayed, Khoudeir and Stigsby, “Intraoperative motor evoked potentials to transcranial electrical stimulation during two anaesthetic regimens” by Pelosi, Stevenson, Hobbs, Jardine and Webb, “The effect of sevoflurane on myogenic motor-evoked potentials induced by single and paired transcranial electrical stimulation of the motor cortex during nitrous oxide/ketamine/fentanyl anesthesia” by Kawaguchi, Inoue, Kakimoto, Kitaguchi, Furuya, Morimoto and Sakaki, “Threshold-level multipulse transcranial electrical stimulation of motor cortex for intraoperative monitoring of spinal motor tracts: description of method and comparison to somatosensory evoked potential monitoring” by Calanci, Harris, Broton, Alexeeva and Green, “Motor evoked potential monitoring during spinal surgery: responses of distal limb muscles to transcranial cortical stimulation with pulse trains” by Jones, Harrison, Koh, Mendoza and Crockard, “Transcranial high-frequency repetitive electrical stimulation for recording myogenic motor evoked potentials with the patient under general anesthesia” by Pechstein, Cedzich, Nadstawek and Schramm, and “Intraoperative Spinal Cord Monitoring” by Houlden. A representative discussion relating to direct electrical stimulation of the spinal cord to elicit MEPs is set forth in “Intra-operative monitoring during surgery for spinal deformity” by Moore and Owen.
Since myogenic MEPs may not indicate motor injury of individual nerve roots, such as the lumbosacral root, it is advantageous in many surgical procedures in which the spinal cord is monitored to also perform neural monitoring involving individual nerve roots. For example, some spinal procedures entail internal fixation with medical devices that may irritate or injure nerve roots, such as the lumbar root, when placed in a patient's body during an operative procedure. Nerve irritation or injury may occur and remain undetected even while standard MEP testing appears normal. It is therefore beneficial to intraoperatively detect dysfunction in individual nerve roots by electrically stimulating the nerve or the anatomical area in the vicinity of the nerve, and monitoring electromyographic (EMG) responses in muscles innervated by the nerve. When electrical stimulation is applied to anatomical tissue at or reasonably near the nerve of interest, the stimulation signal is transmitted through the nerve to excite the related muscle. Excitement of the muscle causes an electrical impulse to be generated within the muscle (EMG) which may be detected by a monitoring or recording electrode in the muscle, thereby providing an indication as to the location and/or integrity of the nerve. Locating a nerve during surgery allows the area of the nerve to be avoided so that it is protected and preserved. Providing an indication of nerve integrity allows nerve irritation or trauma to be detected early, so that the source of irritation or trauma can be identified and corrected. Accordingly, it is beneficial in many types of surgical procedures to perform neural monitoring by monitoring both the spinal cord, using elicited MEPs, and individual nerves/nerve roots, using evoked EMG. The stimulation current for evoked EMG is ordinarily delivered at lower current amperage than the stimulation required to elicit MEPs. In addition to monitoring EMG responses when electrical stimulation is applied, it is also desirable for neural intraoperative monitoring systems to permit neural monitoring involving continuous monitoring of EMG activity from certain muscles at rest and/or when no electrical stimulation is being applied. It would therefore be desirable to provide a single intraoperative neural monitoring system capable of performing multiple modalities of neural monitoring including MEP monitoring and continuous and evoked EMG monitoring.
A representative monitoring or recording electrode for detecting EMG responses in muscles is disclosed in U.S. Pat. No. 5,161,533 to Prass et al. Representative monopolar and bipolar stimulating probes for electrically stimulating a nerve or anatomical tissue in the vicinity of a nerve are disclosed in U.S. Pat. No. 4,892,105 to Prass and U.S. Pat. No. 6,292,701 B1 to Prass et al. Prior nerve integrity monitoring systems for recording EMG activity from muscles and alerting a surgeon when a nerve has been activated by an electric stimulus are represented by U.S. Pat. No. 6,334,068 B1 to Hacker and U.S. Pat. No. 6,306,100 B1 to Prass. The entire disclosures of U.S. Pat. Nos. 4,892,105, 5,161,533, 6,292,701 B1, 6,306,100 B1, and 6,334,068 B1 are incorporated herein by reference.
Prior intraoperative neural monitoring systems are either not designed to provide electrical current of sufficient magnitude to elicit MEPs or are not designed to provide automatic biphasic electrical stimulation sequences between the stimulating electrodes. Biphasic electrical stimulation sequences between stimulating electrodes placed in a patient's body in correspondence with the anatomical areas to be stimulated allow the anatomical areas to be sequentially alternatingly stimulated. Where the stimulated anatomical areas, such as the left and right motor cortex, generate MEPs respectively detectable as EMG responses on opposite sides, i.e. left and right, of the patient's body, unilateral neural deficits can be differentiated. However, where the direction or polarity of current flow between the stimulating electrodes is fixed, providing monophasic electrical stimulation in one direction or polarity between the stimulating electrodes, the anatomical areas cannot be sequentially alternatingly stimulated without manually reversing the locations of the stimulating electrodes with respect to the anatomical areas or electromechanically reversing the lead polarities for the stimulating electrodes each time polarity or direction of current flow between the stimulating electrodes is to be reversed.
The Digitimer D185 Multipulse Stimulator of Digitimer Ltd. allows the direction or polarity of current flow between the stimulating electrodes to be reversed, but not automatically. Rather, a polarity selection switch having “normal” and “reverse polarity” settings must be operated each time the direction or polarity of the stimuli is to be reversed. Operation of the polarity selection switch is in addition to operation of a separate trigger switch that activates the delivery of electrical pulses to the output stimulating electrode. Operation of the trigger switch effects delivery of only one phase (positive or negative) of electrical pulses since the polarity selection switch must be operated in order to deliver pulses of the opposite phase. Another operation of the trigger switch is required to effect delivery of the opposite phase pulses.
Prior intraoperative neural monitoring systems used to elicit MEPs and/or the stimulators used to elicit MEPs are therefore associated with various disadvantages including additional operational steps which increase the duration of the surgical procedures to the detriment of patients and medical personnel, increased complexity and confusion attendant with intraoperative neural monitoring, the possible occurrence of false negative responses due to stimulation on the wrong side of the body, and the need for greater human and/or mechanical intervention. Prior intraoperative neural monitoring systems used to elicit MEPs and/or the stimulators used to elicit MEPs have further drawbacks including failing to provide both positive and negative monophasic and automatic biphasic sequenced outputs from a single stimulator, failing to present left and right EMG waveforms simultaneously and correlated in time to biphasic electrical stimulation to allow a more complete interpretation of neurological motor responses, and the inability to efficiently integrate multiple neural monitoring modalities.